STUDIES AND RESEARCH REGARDING SOUND REDUCTION MATERIALS

WITH THE PURPOSE OF REDUCING SOUND POLLUTION

A Thesis

presented to

the Faculty of California Polytechnic State University,

San Luis Obispo

In Partial Fulfillment

of the Requirements for the

Degree Master of Science in

Architecture

By

Timothy Gerard Hawkins

April 2014

© 2014

Timothy Hawkins

ALL RIGHTS RESERVED

ii

COMMITTEE MEMBERSHIP

TITLE: Studies and Research Regarding Sound Reduction

Materials With The Purpose Of Reducing Sound.

AUTHOR: Timothy Hawkins

DATE SUBMITTED: February 2014

COMMITTEE CHAIR: Roger Phillip Lead TechnicianCal Poly Performing Arts Center

COMMITTEE MEMBER: Michael Haungs Associate Professor, Cal PolyPh.D., UC Davis Computer Science Department

COMMITTEE MEMBER: Jennifer Rennick Lecturer, Cal PolyB.S in Architecture, Cal Poly Architect, CEA

iii

ABSTRACT

Studies and research regarding sound reduction materials

with the purpose of reducing sound.

Timothy Hawkins

Noise can be defined as unwanted sound. There are many cases and applications

that reducing noise level is of great importance. Loss of hearing is only one of the effects

of continuous exposure to excessive noise levels. Noise can interfere with sleep and

speech, and cause discomfort and other non-auditory effects. High level noise and

vibration lead to structural failures as well as reduction in life span. The importance of

noise issue could be well understood by looking at regulations that have been passed by

governments to restrict noise production in society. Industrial machinery, air/surface

transportation and construction activities are main contributors in noise production or

"noise pollution". Noise Pollution is not only an annoyance; it is an environmental health

hazard. Noise can be found anywhere that life exists, in forests, in the workplace, in

homes across America and even under water. A lot of research has been done about noise

pollution in the last 40 years, but yet there is still more to learn about how to control and

lessen the affects that noise has on human and animal life. Noise control is a major factor

in the planning, design, and construction of transportation corridors. Architects,

acoustical engineers and transportation planners are searching for creative ways to

eliminate or greatly reduce noise levels.

iv

The challenge lies in attaining desired sound levels while simultaneously

maintaining or enhancing the visual environment. I will be setting up an experiment to

determine what kinds of materials absorb sound waves of varying frequencies most

effectively.

Keywords: Unwanted sound, Industrial machinery, Acoustical Engineers, Transportation

Planners, Noise Pollution, Noise Control.

v

TABLE OF CONTENTS

LIST OF TABLES viii

LIST OF FIGURES ix

CHAPTER 1 Introduction 1 Statement of the Problem 2

CHAPTER 2 Noise and Sound 3 Definitions and Fundamental Concepts for Sound and Noise 3 Response to the Human Ear and Noise 4 Human Perception of Noise 4 Characteristics of Sound and the Decibel Scale 5 Adding Sounds or Noises Together on the Decibel Scale 10 Propagation of Sound 13

CHAPTER 3 Noise Barriers 14

Sound Attenuation through Noise Barriers 15

CHAPTER 4 Studies and Research regarding Sound Attenuation of Sound Absorbing Materials 18 Parameters of Sound 18 Absorbing Materials 18

CHAPTER 5 Theoretical Basis of Sound Insulation 21

Measurement of Sound 21 Sound Transmission Loss 21

CHAPTER 6 Measurements in the Laboratory 23 Equipment Used in Measurements 23 Sound Proofing 25 Damping 25 Absorption 25 Porous Absorbers 26 Apparatus 27 Frequencies Chosen 27

vi

Materials Thickness Used 28 Experimental Procedure 28 Experimental Assumptions 28

CHAPTER 7 Experimental and Theoretical Results 29 Results 29

CHAPTER 8 Conclusions 30

REFERENCES 31

vii

LIST OF TABLES

Table 1 – Experimental Results 29

viii

LIST OF FIGURES

Figure 1 - Perception of Sound - Human Ear 3

Figure 2 - the Audible Sound Pressure Range is from 0 db to 120d 4

Figure 3 - Sound Pressure Level Expressed in Pa 6

Figure 4 - Common Sound or Noise in Terms of Pa 7

Figure 5 - Decibel Formula 8

Figure 6 - Relation Between Sound Pressure in Decibels vs Micropascals 9

Figure 7 - Formula to Add Three Sounds Together in Decibels 10

Figure 8 - Addition of Sound Levels 11

Figure 9 - Weighting Scale 12

Figure 10 - Noise Barrier berm along Highway 12, Sonoma County, CA 16

Figure 11 - Decibel Meter 23

24

24

Figure 12 - Sound Reduction Index

Figure 13 - Sound Reduction

Materials Figure 14 - Apparatus 27

ix.

1

CHAPTER 1

Introduction

Noise can be defined as unwanted sound. There are many cases and applications

that reducing noise level is of great importance. Loss of hearing is only one of the effects

of continuous exposure to excessive noise levels. Noise can interfere with sleep and

speech, and cause discomfort and other non-auditory effects. High level noise and

vibration lead to structural failures as well as reduction in life span.

The importance of noise issue could be well understood by looking at regulations

that have been passed by governments to restrict noise production in society. Industrial

machinery, air/surface transportation and construction activities are main contributors in

noise production or "noise pollution". Noise Pollution is not only an annoyance; it is an

environmental health hazard. Noise can be found anywhere that life exists, in forests, in

the workplace, in homes across America and even under water.

Statement of the Problem

A lot of research has been done about noise pollution in the last 40 years, but yet

there is still more to learn about how to control and lessen the affects that noise has on

human and animal life. Noise control is a major factor in the planning, design, and

construction of transportation corridors. Architects, acoustical engineers and

transportation planners are searching for creative ways to eliminate or greatly reduce

noise levels. The challenge lies in attaining desired sound levels while simultaneously

maintaining or enhancing the visual environment.

2

I will be setting up an experiment to determine what kinds of materials absorb

sound waves of varying frequencies most effectively. My objective is to compare sound

reduction effectiveness on different materials under different frequencies in term of

Sound Reduction Index(R) and Transmitted coefficient (T).

3

CHAPTER 2

Noise and Sound

Sound is a form of energy that is transmitted by pressure variations which the

human ear can detect. When one plays a musical instrument, say a guitar, the vibrating

chords set air particles into vibration and generate pressure waves in the air. [3] A

person nearby may then hear the sound of the guitar when the pressure waves are

perceived by the ear. Sound can also travel through other media, such as water or steel.

Apart from musical instruments, sound can be produced by many other sources -

man's vocal cord, a running engine, a vibrating loudspeaker diaphragm, an operating

machine tool, and so on. [6]

Definitions and Fundamental Concepts for Sound and Noise

The ear comprises of three parts: The outer ear, the middle ear, and the inner ear.

Figure 1 - Perception of Sound - Human Ear

4

Response to the Human Ear and Noise

Frequency is one of the properties of sound or noise. A sound with a high

frequency is said to be high-pitched and a sound with a low frequency is low-pitched

There is a remarkably wide range of frequencies and sound pressure levels over which

the human ear can detect. The following diagram shows the audible range of a normal

human ear.

Figure 2 - The audible sound pressure range is from 0 dB to 120 dB.

Human Perception of Noise

It is a theory in psychology that our perception of objects, both visual and

auditory, is determined by certain principles. These principles function so that our

perceptual worlds are organized into the simplest pattern consistent with the sensory

information and with our experience. [1]

In hearing, we also tend to organize sounds into auditory objects or streams and

use the principles of grouping to help us to segregate those components we are interested

5

in from others. We are thus able to focus our listening attention to a particular noise

source and distinguish an auditory object from the background noise.

The human ears can detect not only changes in the overall sound pressure level

but are so sophisticated that they can detect sound, the sound pressure level of which is

well below the background noise level.

While there are variations in individual perception of the strength of a sound,

studies have shown that to a good approximation, the sound is perceived twice as loud if

the sound level increases by 10 dB. Similarly, a 20 dB increase in the sound level is

perceived as four times as loud by the normal human ear.

Characteristics of Sound and the Decibel Scale

Sound is the quickly varying pressure wave travelling through a medium. When

sound travels through air, the atmospheric pressure varies periodically. The number of

pressure variations per second is called the frequency of sound, and is measured in Hertz

(Hz) which is defined as cycles per second. [9]

The higher the frequency, the more high-pitched a sound is perceived. The sounds

produced by drums have much lower frequencies than those produced by a whistle, as

shown in the following diagrams. Please click on the demo button to hear their sounds

and the difference in pitch. [9]

Another property of sound or noise is its loudness. A loud noise usually has a

larger pressure variation and a weak one has smaller pressure variation. Pressure and

pressure variations are expressed in Pascal, abbreviated as Pa, which is defined as

N/m2(Newton per square metre).

6

Human ear can perceive a very wide range of sound pressure. The softest sound a

normal human ear can detect has a pressure variation of 20 micro Pascals, abbreviated as

µPa, which is 20 x 10-6 Pa ("20 millionth of a Pascal") and is called the Threshold of

Hearing. On the other hand, the sound pressure close to some very noisy events such as

launching of the space shuttle can produce a large pressure variation at a short distance of

approximately 2000 Pa or 2 x 109 µPa. [1]

The following table illustrates sound pressure level of the above events expressed

in Pa and µPa.

Sound Pressure expressed in

Pa µPa

Softest Noise just Heard by a

Human Ear 20 x 10-6 20

Launching of the Space Shuttle 2,000 2 x 109

Figure 3 – Sound Pressure Level Expressed in Pa

7

To express sound or noise in terms of Pa is quite inconvenient because we have to

deal with numbers from as small as 20 to as big as 2,000,000,000. [2] The following table

shows some common sound or noise in terms of µPa:

Source of Sound/Noise

Approximate Sound

Pressure

in µPa

Launching of the Space Shuttle 2,000,000,000

Full Symphony Orchestra 2,000,000

Diesel Freight Train at High Speed at 25 m 200,000

Normal Conversation 20,000

Soft Whispering at 2 m in Library 2,000

Unoccupied Broadcast Studio 200

Softest Sound Human can Hear 20

Figure 4 – Common Sound or Noise in terms of Pa

8

A simpler way is to use a logarithmic scale for the loudness of sound or noise,

using 10 as the base. The following is a brief introduction of the common logarithm to

the base 10.

To avoid expressing sound or noise in terms of Pa, which could involve some

unmanageable numbers, the decibel or dB scale is used. The scale uses the hearing

threshold of 20 μPa or 20 x 10-6 Pa as the reference level. This is defined as 0 dB. Sound

pressure level, which is often abbreviated as SPL or Lp, in decibels (dB), can then be

obtained using the following formula.

Figure 5 – Decibel Formula

9

In the following chart, some sounds are expressed both linearly in µPa and

logarithmically in dB. One can see how the logarithmic scale helps us to handle numbers

on a wide scale much more easily.

Figure 6 –Relation Between Sound Pressure in Decibels vs Micropascals

10

One useful aspect of the decibel scale is that it gives a much better approximation

to the human perception of relative loudness than the Pascal scale. This is because

human ear responds to the logarithmic change in level, which corresponds to the decibel

scale. [6]

Adding Sounds or Noises together on the Decibel Scale

In real life, several sources of sounds often occur at the same time. One may be

interested to know what results when one sound is combined with another, i.e. the

addition of sounds. [10]

Adding 60 apples to 60 apples results in 120 apples. But this is not the case with

sounds when they are expressed in decibels. In fact, adding 60 decibels to 60 decibels

gives 63 decibels. The following formula explains the general principle of adding sounds

on the decibel scale.

Figure 7 – Formula to add three sounds together in Decibels

11

Addition of sound levels can also be done simply using the following chart

Figure 8 – Addition of Sound Levels

In using the Chart, two sounds are added together first. The resultant sound is then

added to a third sound and so on.

A normal human ear is able to hear sounds with frequencies from 20 Hz to 20,000

Hz. The range of 20 Hz to 20,000 Hz is called the audible frequency range. The sounds

we hear comprise of various frequencies. The entire audible frequency range can be

divided into 8 or 24 frequency bands known as octave bands or 1/3 octave bands

respectively for analysis. A particular sound or noise can be seen to be having different

strengths or sound pressure levels in the frequency bands, as illustrated by the following

diagram.

12

One single sound pressure level is often used to describe a sound. This can be

done by adding the contribution from all octave bands or 1/3 octave bands together to

yield one single sound pressure level.

The response of the ear to sound is dependent on the frequency of the sound. The

human ear has peak response around 2,500 to 3,000 Hz and has a relatively low response

at low frequencies. Hence, the single sound pressure level obtained by simply adding the

contribution from all octave bands or 1/3 octave bands together will not correlate well

with the non-linear frequency response of the human ear.

This has led to the concept of weighting scales. The following diagram shows the

"A-weighting" scale:

Figure 9 –Weighting Scale

13

In the "A-weighting" scale, the sound pressure levels for the lower frequency

bands and high frequency bands are reduced by certain amounts before they are being

combined together to give one single sound pressure level value. This value is designated

as dB(A). The dB(A) is often used as it reflects more accurately the frequency response

of the human ear. Weighting networks are often incorporated in measuring equipments to

give readings in dB(A).

Propagation of Sound

In air, sound is transmitted by pressure variations from its source to the

surroundings.

The sound level decreases as it gets further and further away from its source. While

absorption by air is one of the factors attributing to the weakening of a sound during

transmission, distance plays a more important role in noise reduction during transmission.

The reduction of a sound is called attenuation. The effect of distance attenuation

depends on the type of sound sources. Most sounds or noises we encountered in our daily

life are from sources which can be characterized as point or line sources. If a sound

source produces spherical spreading of sound in all directions, it is a point source. [3]

For a point source, the noise level decreases by 6 dB per doubling of distance

from it. If the sound source produces cylindrical spreading of sound as shown in this

diagram, such as stream of motor vehicles on a busy road at a distance, it may be

considered as a line source. Please click on the demo button to see the details. For a line

source, the noise level decreases by 3 dB per doubling of distance from it. [3]

14

CHAPTER 3

Noise Barriers

A noise barrier (also called a soundwall, sound berm, sound barrier, or acoustical barrier)

is an exterior structure designed to protect inhabitants of sensitive land use areas from

noise pollution. Noise barriers are the most effective method of mitigating roadway,

railway, and industrial noise sources – other than cessation of the source activity or use of

source controls.

In the case of surface transportation noise, other methods of reducing the source

noise intensity include encouraging the use of hybrid and electric vehicles, improving

automobile aerodynamics and tire design, and choosing low-noise paving material.

Extensive use of noise barriers began in the United States after noise regulations were

introduced in the early 1970s.

Noise barriers have been built in the United States since the mid-twentieth

century, when vehicular traffic burgeoned. In the late 1960s, acoustical science

technology emerged to mathematically evaluate the efficacy of a noise barrier design

adjacent to a specific roadway. By the 1991s, noise barriers that included use of

transparent materials were being designed in Denmark and other western European

countries. [11]

The best of these early computer models considered the effects of roadway

geometry, topography, vehicle volumes, vehicle speeds, truck mix, roadway surface type,

and micro-meteorology. Several U.S. research groups developed variations of the

computer modeling techniques: Caltrans Headquarters in Sacramento, California; the

ESL Inc. group in Palo Alto, California; the Bolt, Beranek and Newman group in

15

Cambridge, Massachusetts, and a research team at the University of Florida. Possibly the

earliest published work that scientifically designed a specific noise barrier was the study

for the Foothill Expressway in Los Altos, California. [13]

Numerous case studies across the U.S. soon addressed dozens of different existing

and planned highways. Most were commissioned by state highway departments and

conducted by one of the four research groups mentioned above. The U.S. National

Environmental Policy Act [7] effectively mandated the quantitative analysis of noise

pollution from every Federal-Aid Highway Act Project in the country, propelling noise

barrier model development and application. With passage of the Noise Control Act of

1972, demand for noise barrier design soared from a host of noise regulation spinoff.

By the late 1970s, more than a dozen research groups in the U.S. were applying similar

computer modeling technology and addressing at least 200 different locations for noise

barriers each year. As of 2006, this technology is considered a standard in the evaluation

of noise pollution from highways. The nature and accuracy of the computer models used

is nearly identical to the original 1970s versions of the technology. [13]

Sound Attenuation through noise Barriers

The acoustical science of noise barrier design is based upon treating an airway or

railway as a line source. The theory is based upon blockage of sound ray travel toward a

particular receptor; however, diffraction of sound must be addressed. Sound waves bend

(downward) when they pass an edge, such as the apex of a noise barrier. Further

complicating matters is the phenomenon of refraction, the bending of sound rays in the

16

presence of an inhomogeneous atmosphere. Wind shear and thermocline produce such

inhomogeneities. [16]

The sound sources modeled must include engine noise, tire noise, and

aerodynamic noise, all of which vary by vehicle type and speed. The resulting computer

model is based upon dozens of physics equations translated into thousands of lines of

computer code. Software applications are available which are able to model these

situations and assist in the design of such noise barriers.

Figure 10 – Noise Barrier berm along Highway 12, Sonoma County, California

Noise barrier earth berm along Highway 12, Sonoma County, California

Some noise barriers consist of a masonry wall or earthwork, or a combination thereof

(such as a wall atop an earth berm). Sound abatement walls are commonly constructed

using steel, concrete, masonry, wood, plastics, insulating wool, or composites. In the

most extreme cases, the entire roadway is surrounded by a noise abatement structure, or

dug into a tunnel using the cut-and-cover method. The noise barrier may be constructed

on private land, on a public right-of-way, or on other public land. Because sound levels

are measured using a logarithmic scale, a reduction of nine decibels is equivalent to

elimination of approximately 80 percent of the unwanted sound. [14]

17

Noise barriers can be extremely effective tools for noise pollution abatement, but

theory calculates that certain locations and topographies are not suitable for use of any

reasonable noise barrier. Cost and aesthetics play a role in the final choice of any noise

barrier. [12]

Normally, the benefits of noise reduction far outweigh aesthetic impacts for

residents protected from unwanted sound. These benefits include lessened sleep

disturbance, improved ability to enjoy outdoor life, reduced speech interference, stress

reduction, reduced risk of hearing impairment, and a reduction in the elevated blood

pressure created by noise that improves cardiovascular health. [15]

18

CHAPTER 4

Studies and Research regarding Sound Attenuation of Sound Absorbing Materials

All materials have some sound absorbing properties. Incident sound energy which

is not absorbed must be reflected, transmitted or dissipated. A material’s sound absorbing

properties can be described as a sound absorption coefficient in a particular frequency

range. The coefficient can be viewed as a percentage of sound being absorbed, where

1.00 is complete absorption (100%) and 0.01 is minimal (1%). [4]

Parameters of Sound

Incident sound striking a room surface yields sound energy comprising reflected

sound, absorbed sound and transmitted sound. Most good sound reflectors prevent sound

transmission by forming a solid, impervious barrier. Conversely, most good sound

absorbers readily transmit sound. Sound reflectors tend to be impervious and massive,

while sound absorbers are generally porous, lightweight material. It is for this reason that

sound transmitted between rooms is little affected by adding sound absorption to the wall

surface.

Absorbing Materials

There are three basic categories of sound absorbers: porous materials commonly

formed of matted or spun fibers; panel (membrane) absorbers having an impervious

surface mounted over an airspace; and resonators created by holes or slots connected to

an enclosed volume of trapped air. The absorptivity of each type of sound absorber is

dramatically (in some cases) influenced by the mounting method employed. [8]

19

1) Porous absorbers: Common porous absorbers include carpet, draperies, spray-applied

cellulose, aerated plaster, fibrous mineral wool and glass fiber, open-cell foam, and felted

or cast porous ceiling tile. Generally, all of these materials allow air to flow into a cellular

structure where sound energy is converted to heat. Porous absorbers are the most

commonly used sound absorbing materials. Thickness plays an important role in sound

absorption by porous materials. Fabric applied directly to a hard, massive substrate such

as plaster or gypsum board does not make an efficient sound absorber due to the very thin

layer of fiber. Thicker materials generally provide more bass sound absorption or

damping. [5]

2) Panel Absorbers: Typically, panel absorbers are non-rigid, non-porous materials which

are placed over an airspace that vibrates in a flexural mode in response to sound pressure

exerted by adjacent air molecules. Common panel (membrane) absorbers include thin

wood paneling over framing, lightweight impervious ceilings and floors, glazing and

other large surfaces capable of resonating in response to sound. Panel absorbers are

usually most efficient at absorbing low frequencies. This fact has been learned repeatedly

on orchestra platforms where thin wood paneling traps most of the bass sound, robbing

the room of “warmth.” [14]

3) Resonators: Resonators typically act to absorb sound in a narrow frequency range.

Resonators include some perforated materials and materials that have openings (holes

and slots). The classic example of a resonator is the Helmholtz resonator, which has the

shape of a bottle. The resonant frequency is governed by the size of the opening, the

length of the neck and the volume of air trapped in the chamber. Typically, perforated

20

materials only absorb the mid-frequency range unless special care is taken in designing

the facing to be as acoustically transparent as possible. Slots usually have a similar

acoustic response. Long narrow slots can be used to absorb low frequencies. For this

reason, long narrow air distribution slots in rooms for acoustic music production should

be viewed with suspicion since the slots may absorb valuable low-frequency energy. [14]

21

CHAPTER 5

Theoretical Basis of Sound Insulation

The sound insulation or sound transmission loss of a wall is that property which

enables it to resist the passage of noise or sound from one side to the other. This should

not be confused with sound absorption which is that property of a material which permits

sound waves to be absorbed, thus reducing the noise level within a given space and

eliminating echoes or reverberations.

Measurement of Sound

The sound insulation of a building assembly is expressed as a reduction factor in decibels

(dB). The decibel is approximately the smallest change in energy the human ear can

detect, and the decibel scale is used for measuring ratios of sound intensities. The

reference sound intensity used to measure absolute noise levels is that corresponding to

the faintest sound a human ear can hear (0 dB). However, a difference of 3 or less dB is

not especially significant, because the human ear cannot detect a change in sounds of less

than 3 dB.

Sound Transmission Loss

It is desirable to have a single number rating as a means for describing the

performance of building elements when exposed to an "average" noise. In the past it was

customary to use the numerical average of the transmission loss values at nine

frequencies. This rating, termed the nine-frequency average transmission loss, is often

quite inaccurate in comparing an assembly of materials having widely differing TL-

22

frequency characteristics. One single number rating method which has been recently

proposed is the sound transmission class (STC). This rating is based on the requirements

that the value of transmission loss at any of the eleven measuring frequencies does not

fall below a specified TL-frequency contour. The shape of this contour is drawn to

represent

The more common types of noise, and generally covers the requirements for

speech privacy.

23

CHAPTER 6

Measurements in the Laboratory

I decided to use materials that are commonly seen. Then I will compare sound insulation

effectiveness on the different materials under different frequencies in term of Sound

Reduction Index(R) and Transmitted coefficient (T).

Equipment Used in Measurements

Decibel Meter – SPL Meter Ipad Application: Using a decibel meter we can measure the

sound level in (db) performance for each material in the experiment.

Figure 11 – Decibel Meter

24

Figure 12 –Sound Reduction Index

Expandable Polystyrene Stainless Steel

Wood Paper

Figure 13 –Sound Reduction Materials

Sound Proofing

Soundproofing is any means of reducing the sound pressure with respect to a

specified sound source and receptor. There are several basic approaches to reducing

sound: increasing the distance between source and receiver, using noise barriers to reflect

or absorb the energy of the sound waves, using damping structures such as sound baffles,

or using active anti noise sound generators.

I will be using sound barriers to absorb the energy of the sound waves. The

energy density of sound waves decreases as they spread out, so that increasing the

distance between the receiver and source results in a progressively lesser intensity of

sound at the receiver. In a normal three dimensional setting, with a point source and point

receptor, the intensity of sound waves will be attenuated according to the inverse square

of the distance from the source. I will use the same distance for each test.

Damping

Damping means to reduce resonance in the room, by absorption or redirection

(reflection or diffusion). Absorption will reduce the overall sound level, whereas

redirection makes unwanted sound harmless or even beneficial by reducing coherence.

Damping can reduce the acoustic resonance in the air, or mechanical resonance in the

structure of the room itself or things in the room.

Absorption

Absorbing sound spontaneously converts part of the sound energy to a very small

amount of heat in the intervening object (the absorbing material), rather than sound being

25

transmitted or reflected. There are several ways in which a material can absorb sound.

The choice of sound absorbing material will be determined by the frequency distribution

of noise to be absorbed and the acoustic absorption profile required.

Porous Absorbers

Porous absorbers, typically open cell rubber foams or melamine sponges, absorb

noise by friction within the cell structure. Porous open cell foams are highly effective

noise absorbers across a broad range of medium-high frequencies. Performance is less

impressive at low frequencies.

The exact absorption profile of a porous open cell foam will be determined by a number

of factors including the following:

• Cell size

• Tortuosity

• Porosity

• Material thickness

• Material density

26

27

Apparatus

Figure 14 –Apparatus

Frequencies Chosen

F1 – 125 Hz

F2 – 250 Hz

F3 – 500 Hz

F4 - 1 kHz

F5 – 2 kHz

28

Materials Thickness Used

EPS FOAM .7” THICKNESS

STEEL BOARD .08” THICKNESS

WOODEN BOARD .47” THICKNESS

PAPER (50 SHEETS) .20” THICKNESS

PAPER (100 SHEETS) .20” THICKNESS

Experimental Procedure

1. Turn on speaker and generate 125Hz

2. Measure sound level using SPL meter and record the readings

3. Using EPS Foam in front of box so that it covers the opened part of the box

4. Measure sound level and take 3 readings.

5. Repeat steps 1 thru 4 for the other materials in the experiment

6. calculate the average sound level (Laverage = ((L1+L2+L3)/3), Sound reduction

index ® and transmitted coefficient (T) for different material under different

frequency.

7. Show relation between R/T and different frequencies for different materials

Experimental Assumptions

1. The unique sound source for the experiment is the frequency generator

2. Backround noise is kept constant

3. The reflection and refraction effects of sound are negligible

29

CHAPTER 7

Experimental and Theoretical Results

Materials F1 F2 F3 F4 F5 Sound Level

125Hz 250Hz 500Hz 1kHz 2kHz

EPS Foam 60 71 70 75 77 L(average) (dBb)

7” Thick 3.2 1.4 1.5 .50 5.2 R(db)

.5 .7 .7 .9 .3 T

Steel Board 62 59 53 53 55 L(average) (dBb)

.08” Thick 5.5 9.0 5.0 15.0 19.2 R(db)

.3 .2 .4 .02 .01 T

Wooden Board 69 68 57 56 51 L(average) (dBb)

.47” Thick 3.5 4.0 5.0 10.9 22. R(db)

.45 .4 4.6 10.9 22.0 T

A4 Paper 57 75 75 69 72 L(average) (dBb)

.20” Thick 1.5 1.0 1.0 2.0 2.2 R(db)

.7 .8 .8 .6 .6 T

A4 Paper 54 65 75 64 69 L(average) (dBb)

.40” Thick 1.8 2.6 1.7 6.5 5.5 R(db)

.7 .5 .7 .3 .3 T

Table 1 –Experimental Results

30

CHAPTER 8

Conclusions

1. The greater the R, the greater the sound reduction level.

2. The smaller the T, the greater the sound reduction level.

3. The most effective sound insulation material is the steel board.

4. the least effective sound insulation materials are the EPS foam and A4

Paper(50sheets).

31

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[4] Maxwell, Clinton. "Cool Ideas: How Sound Insulations Work." Cool Ideas. N.p.,

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[5] "Products | Acoustical Solutions, Inc. | Soundproofing and Noise Control

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[6] Schmid, Randolph E. “Aging Nation Faces Growing Hearing Loss” 18 February

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[7] "NPC Law Library: Noise Regulations & Ordinances From The 50 Biggest

Cities." NPC Law Library: Noise Regulations & Ordinances From The 50 Biggest

Cities. N.p., 29 Jan. 2009. Web. 30 May 2013..

Orellana, Judith. Personal Interview. 31 January 2009

[8] Cox, Trevor J., and Peter D'Antonio. Acoustic Absorbers and Diffusers: Theory,

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[9] "Acoustics." Acoustics. N.p., July 2011. Web. 30 May 2014.

32

[10] Buckley, Cara. "U.S. Standards on Workplace Noise Trail Those of Other

Countries." The New York Times. The New York Times, 19 July 2012. Web. 30

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